Material Design for High-T_c Superconductors by MBE and Study for Electronic State Using Cross Sectional STM

人们普遍认为，高t_c超导氧化物具有由CuO_2层导电块和电荷储层块组成的固有超结构。本课题研究利用分子束外延（MBE）在原子尺度上生长高t_c氧化物的异质结构。我们研究了PrBa_2Cu_3O_<7- δ > (PrBCO) /YBa_2Cu_3O_<7- δ > (YBCO) /PrBCO和SrTiO_3/SrCuO_2异质结构。我们发现，当末端CuO_<1 δ >层被氧化物（MO岩盐型层）覆盖，形成MO- cuo - bao电荷储层时，c轴取向的YBCO表面1单元胞层呈现超导性。这一结果对隧道结和电场器件具有重要意义。我们成功地利用电场改变了具有无限层结构的SrCuO_2薄膜的电阻率。通过扫描隧道显微镜（STM）观察发现，YBCO和SrRuO_3分别以二维单细胞逐单细胞生长方式和阶梯流生长方式生长。对设计的异质结构进行透射电镜观察，揭示了电子结构和载流子掺杂成导电层的机理。我们成功地生长了由超薄层高t_c超导氧化物组成的异质结构。近年来，利用约瑟夫森等离子体振荡的毫米波发生器在高t_c超导体中有很好的应用前景，引起了人们的广泛关注。我们的研究结果将有助于实现这种器件的制造。

It is widely accepted that high-T_c superconducting oxides have the inherent superstructures comprising of conducting blocks based on CuO_2 layrs and charge reservoir blocks. This subject concerns the growth of heterostructures of high-T_c oxides on atomic scales by molecular beam epitaxy (MBE) for the material design. We have studied PrBa_2Cu_3O_<7-delta> (PrBCO) /YBa_2Cu_3O_<7-delta> (YBCO) /PrBCO and SrTiO_3/SrCuO_2 heterostructures.We have found that the surface one unit cell layr of c-axis oriented YBCO shows a superconductivity when the terminating CuO_<1-delta> layr has covered with an oxide, which has MO rocksalt-type layr, to form MO-CuO-BaO charge reservoir block. This result is of primary importance for the tunneling junctions and electric field devices.We have succeeded the change of electrical resistivity for SrCuO_2 thin films with infinite layr structure by applying electric field.It has been found through scanning tunneling microscope (STM) observation that YBCO and SrRuO_3 grow by a two-dimensional unit cell by unit cell growth manner and a step flow growth manner, respectively. Cross sectional TEM observation for designed heterostructures would reveal the electronic structures and carrier doping mechanism into conducting layrs.We have successfully grown heterostructures consisting of ultrathin layrs of high-T_c superconducting oxides. Recently, milli-meter wave generator using Josephson plasma oscillations have attracted much attentions for a promising applications of high-T_c superconductors. Our result would contribute to realize the fabrication of such devices.